The present invention relates to an alkaline storage battery using a negative electrode of a hydrogen absorbing alloy.
Recently, hydrogen absorbing alloys capable of electrochemically absorbing/desorbing a great deal of hydrogen serving as an active material have attracted attention as an electrode material of high energy density and have been intended to be applied to a closed alkaline storage battery to be developed into a high capacity storage battery, in particular, to be applied to a closed nickel-hydrogen storage battery.
The electrode reaction in such a closed nickel-hydrogen storage battery is as follows. ##STR1## In the reaction equation (2), M represents a hydrogen absorbing alloy.
A hydrogen absorbing alloy negative electrode for use in this type storage battery is prepared by a process in which an alkali-resisting organic high molecule, such as polyethylene, fluorocarbon polymer, or the like, is added as a binding agent to a pulverized hydrogen absorbing alloy, and the resulting mixture is pressed onto or filled into an electrically conductive collector such as a punching metal or a foam metal.
When the battery is overcharged, gas generation reactions represented by the following equations (3) and (4) occur on the positive electrode and the negative electrode of the battery, respectively. ##STR2## To suppress the increase of the battery inner pressure, a method in which an oxygen gas generated from the positive electrode according to the equation (3) is made to react with hydrogen absorbed in the negative electrode to thereby generate water has been employed. To suppress the generation of a hydrogen gas according to the equation (4), a method in which the capacity of the negative electrode is established to be larger than the capacity of the positive electrode has been employed.
When the battery is charged rapidly, however, the rate of generation of an oxygen gas is often larger than the rate of absorption of the same gas so that the oxygen gas is accumulated in the battery to thereby increase the inner pressure of the battery. To eliminate the aforementioned disadvantage, a method of accelerating reduction of an oxygen gas by adding a noble metal catalyst such as platinum to the negative electrode (as disclosed in Japanese Patent Unexamined Publication No. 60-100382), a method of accelerating absorption of an oxygen gas onto the negative electrode by providing a hydrophobic layer in the hydrogen absorbing alloy negative electrode (as disclosed in Japanese Patent Unexamined Publication No. 61-118963), and the like, are known.
However, various problems arise in the aforementioned, conventional construction of the battery as follows. The method of adding a noble metal to the negative electrode has a problem in that the material cost is increased. On the other hand, the method of providing a hydrophobic layer in the negative electrode has a problem in that the discharging voltage is dropped because of the uneven electrolyte distribution of the negative electrode and the decrease of the effective surface area for the electrochemical reaction. Further, the aforementioned method is effective for improving the oxygen absorption capacity of the negative electrode but has another problem in that the inner pressure of the battery is increased because hydrogen is to be generated from the negative electrode in charging the battery with lowering of the wetting property for the electrolytic solution, of the inside of the hydrogen absorbing alloy negative electrode. In particular, this fact is remarkable when the battery is charged rapidly.